In the electrolysis industry, the reduction of energy consumption is of great importance. In particular, a great deal of attention has been directed to lowering the bath voltage.
Combined with the industrialization of brine electrolysis using an ion exchange membrane, the use of an insoluble metal electrode made of a noble metal oxide as an anode can almost completely eliminate primary factors responsible for the rise in bath voltage. On the other hand, low carbon steel, which has heretofore been used as a cathode material, results in a hydrogen overvoltage considerably as high as 300 to 400 mV. Substitutes for this carbon steel that have recently been used include stainless steel, nickel and a nickel-plated material. However, these substitutes leave something to be desired in achieving the object of lowering the hydrogen overvoltage.
In order to lower the overvoltage by increasing surface area, some approaches have been attempted such as a method involving the elution of certain components from an alloy deposit, a method involving plasma spraying of a particle material and a method involving suspension plating. However, the cathode obtained by these methods is disadvantageous in that it has a roughened surface that can damage the ion exchange membrane and leaves something to be desired in the effect of lowering the bath voltage.
A method for lowering the cathode overvoltage that has recently been mainly used involves coating a nickel substrate with a platinum metal or oxide thereof as a catalyst component. Known examples of coating with a platinum metal oxide include a method which comprises applying a solution containing a platinum metal oxide to a heated metal substrate, and then calcining the coated material to form an oxide of ruthenium or the like on the surface thereof (JP-B-55-22556 (The term "JP-B" as used herein means an "examined Japanese patent application")), a method which comprises attaching a powder of an oxide of ruthenium or the like to the surface of a substrate with nickel by suspension plating (JP-B-59-48872, JP-B-60-13074), and a method which involves forming a composite oxide of metals such as nickel and ruthenium (JP-A-59-232284 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). The cathode prepared by these methods provides a low hydrogen overvoltage which is hardly affected by impurities such as iron in the electrolyte. However, since these methods involve the use of an unstable oxide as a cathode, the resulting cathode has insufficient durability and thus is disadvantageous in that it often operates for a reduced period of time.
On the other hand, a cathode is known comprising a platinum metal, particularly platinum or an alloy thereof, chemically deposited on a substrate made of nickel or the like (JP-A-57-23083). This cathode provides a low hydrogen overvoltage and has a high durability, but is disadvantageous in that it is liable to become poisoned by impurities such as iron in the electrolyte. In other words, such a platinum-coated cathode is very sensitive to impurities in the electrolyte, particularly iron ion. Accordingly, the platinum-coated cathode can lose its low hydrogen overvoltage activity even in the presence of iron ion in an amount of as small as not more than 1 ppm. However, because most electrolysis apparatus and its piping are formed from an iron-containing material, it is extremely difficult to avoid the presence of iron ion in the electrolyte, unavoidably causing deterioration of the cathode.
In order to overcome these difficulties, a cathode for electrolysis has been proposed comprising a catalyst layer coating the cathode containing at least one of a platinum metal, a platinum metal oxide and a platinum metal hydroxide and at least one of cerium, cerium oxide and cerium hydroxide (JP-B-6-33492). In general, cerium is chemically active and thus can hardly be present in a caustic soda solution. Further, since cerium has a poor electrical conductivity, it can easily add to the resistance of the foregoing coating layer. Thus, cerium has been said to be impractical as a cathode catalyst for the electrolysis of brine. However, when mixed with the foregoing platinum metal components to provide a composite coating layer, the cerium component can be extremely stable in a high concentration alkali to obtain a low hydrogen overvoltage cathode coat having both excellent durability and resistance to poisoning and sufficient electrical conductivity. This is presumably because the cerium component in the coating layer forms a cerium hydroxide difficultly soluble in a high concentration alkali and adds to the overvoltage for the reaction of deposition of iron on the platinum metal component.
However, since the above cathode having high activity and resistance to poisoning by iron is coated only with a porous catalyst layer on the substrate thereof, it leaves something to be desired in the adhesivity between the catalyst coat and the substrate. Accordingly, the catalyst coat containing a platinum metal component and a cerium component can be peeled off from the substrate or can partially fall off from the substrate. When these defects occur, the substrate can be exposed to the high concentration aqueous solution of alkali to undergo corrosion, considerably reducing the electrode life. Further, the substrate can be dissolved in the high concentration aqueous solution of alkali, adding to the content of contaminants in the product.